Most of the major relational database management systems (RDBMSs) allow for a DISTINCT clause to be included in our SQL queries.
We use the DISTINCT keyword to return only unique rows. It eliminates duplicates from the results. If we have two or more rows with exactly the same data, we’ll only see one row in the results.
In Oracle Database, we can use the OFFSET clause to make a SELECT statement skip a certain number of rows in its result set.
The OFFSET clause is defined as part of the FETCH row limiting clause, which allows us to limit the number of rows returned by a query.
We can therefore use both OFFSET and FETCH to limit the output to just the specified number or percentage of rows, at a specified offset.
If you’re getting an error that reads “A TOP can not be used in the same query or sub-query as a OFFSET” when running a query in SQL Server, it’s probably because you’re using the TOP() clause and the OFFSET clause in the same query or sub-query.
We can’t use the TOP() clause and the OFFSET clause in the same query in SQL Server.
Below are three options for fixing this error.
The LAG() function is a window function that’s available in many SQL databases. It returns the value of a given expression from the row that lags (precedes) the current row by a given number of rows within its partition.
In other words, the LAG() function returns a value from a previous row.
Many relational database management systems (RDBMSs) have a LEAD() function that allows us to retrieve a value from a following row.
The SQL LEAD() function returns the value of a given expression from the row that leads (follows) the current row by a given number of rows within its partition.
Some of the major relational database management systems (RDBMSs) have a LIMIT clause that enables us to reduce the number of rows returned by a query.
The way it works is that we provide the number of rows we want to be returned by the query. We can also provide an offset to specify which row to start the count from.
If you’re getting an error that reads “The function ‘LAG’ must have an OVER clause” in SQL Server, it’s probably because you’re calling the LAG() function without an OVER clause.
The LAG() function requires an OVER clause (and that clause must have an ORDER BY clause).
To fix this issue, be sure to include an OVER clause when calling the LAG() function.
Many RDBMSs include both a rank() and a dense_rank() function in their list of window function offerings. At first glance, these functions might appear to do the same thing. However, there’s one important difference between these functions, and you will definitely need to be aware of this difference when choosing which function to use.
Oracle Database has a FETCH clause that allows us to limit the number of rows returned by a SELECT statement.
We can use this clause to specify the exact number of rows or the percentage of rows that a query should return. We can also specify an offset for which to start.
The SQL ROW_NUMBER() function is a window function that assigns and returns a row number of each row in a query partition or result set. Numbering starts at 1 and increments sequentially.
This enables us to add a “row number” column to our queries.